1. Field of the Invention
The present invention relates to a bearing and a rotary anode X-ray tube employing the bearing, and particularly to a bearing that can maintain a stable lubrication state under the conditions of high temperature and vacuum.
2. Description of the Prior Art
FIG. 1 is a sectional view showing a conventional ball bearing 3. The ball bearing 3 is disposed between a bearing fitting face 1 and a rotary shaft 2. The ball bearing 3 comprises an inner race 4, an outer race 5 and a plurality of balls 6 rotatably arranged between the inner and outer races 4 and 5. Frictional surfaces of the inner and outer races 4 and 5 and balls 6 are coated with a proper lubricant (not shown).
When an apparatus employing such a ball bearing 3 is used under the conditions of high temperature and vacuum, the ball bearing 3 is heated to a high temperature. To realize good lubrication under such conditions, lead (Pb) and silver (Ag) are conventionally used as the lubricant.
If lead is used as the lubricant on the frictional surfaces of the ball bearing 3, an evaporating rate of the lead increases as the temperature of the ball bearing 3 increases. As a result, a high vacuum may not be maintained with the lead. If silver is used as the lubricant, frictional conditions of the ball bearing 3 become more severe when the ball bearing 3 is rotated at high speed at high temperatures, because the silver is hard. As a result, stable lubrications may not be maintained and a service life of the ball bearing 3 may be shortened with the silver.
FIG. 2 is a sectional view schematically showing a rotary anode X-ray tube according to the prior art. A vacuum vessel 10 contains a cathode 11 for emitting electrons, a rotary shaft 12, a rotor 13 fixed to the rotary shaft 12, and an anode target 14 fixed to the rotor 13. The cathode 11 and anode target 14 face each other. The rotary shaft 12 is rotatably supported by a support shaft 16 through two ball bearings 15a and 15b. Each of the ball bearings 15a and 15b comprises an inner race 17, an outer race 18, and a plurality of balls 19 rotatably arranged between the inner and outer races 17 and 18. A magnetic field generator (not shown) is disposed outside the vacuum vessel 10 to generate a rotating magnetic field that rotates the rotary shaft 12 together with the rotor 13, thus rotating the anode target 14.
When electrons emitted from the cathode 11 of the rotary anode X-ray tube hit the anode target 14, X-rays are generated to heat the anode target 14 and the inside of the vacuum vessel 10. When the anode target 14 and vacuum vessel 10 are heated to high temperatures, the ball bearings 15a and 15b are also heated due to heat radiation and heat conduction from the rotary shaft 12. To prevent the ball bearings 15a and 15b from seizure and wearing due to the heat, the frictional surfaces of the inner and outer races 17 and 18 and balls 19 of the ball bearings 15a and 15b are coated with the lubricant. Since the bearings are used under the conditions of high temperature and vacuum, oil lubrication is not possible to take place. Therefore, a solid lubricant such as lead and silver is used as mentioned in the above.
To increase an output of the rotary anode X-ray tube, electrons hitting the anode target 14 shall be increased. Then, the anode target 14 is further heated to increase the temperatures of the ball bearings 15a and 15b. To prevent the anode target 14 from locally heating and melting due to the colliding electrons, the anode target 14 shall be rotated at high speed.
If the temperature of the anode target 14 increases due to the increased output, an evaporating rate of lead, if the lead is used as the lubricant of the frictional surfaces of the ball bearings 15a and 15b, increases so that a high vacuum may not be maintained. Namely, if the lead is used as the lubricant, it may be usable up to about 300 degrees centigrade but it may not be usable if the temperature increases to about 400 degrees centigrade, because the vapor pressure of lead is high.
On the other hand, if the silver is used as the lubricant, friction of the ball bearings 15a and 15b may increase because the silver is hard, so that stable lubrication may not be maintained, and service lives of the bearings shortened.
As described in the above, the conventional bearings use lead and silver as a lubricant for frictional surfaces that are subjected to high temperatures and vacuum. Since the lead and silver lower the degree of vacuum or enlarge friction, stable lubrication is not maintained. In addition, the rotary anode X-ray tube employing lead or silver as lubricant for frictional surfaces of its bearings may cause a decrease in the vacuum degree or an increase in friction if a temperature increases, to deteriorate lubrication of the bearings.